> What happens when biotech enables effectively (or in practice, externally undetectable) seamless integration of artificial storage into the brain?

I would[2] put good money on "will never happen"[1].

[1] Or at least "any chance of it happening are on a timescale long enough that human civilisation will ruin itself well before we get there".

[2] But for the fact this is a long term bet and I am already old.